Go proxy server

main.go

package main

import (
	"flag"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
)

func copyHeader(to, from http.Header) {
	for hdr, items := range from {
		for _, item := range items {
			to.Add(hdr, item)
		}
	}
}

func extractPath(uri string) string {
	u, err := url.Parse(uri)
	if err != nil {
		return "/"
	}
	return strings.Split(uri, u.Host)[1]
}

type Proxy struct{}

var prefix, replacement string

func preProcess(r *http.Request) {
	if prefix != "" && strings.HasPrefix(r.RequestURI, prefix) {
		newUri := strings.Replace(r.RequestURI, prefix, replacement, 1)
		r.RequestURI = newUri
		u, _ := url.Parse(newUri)
		r.URL = u
	}
}

func (p Proxy) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	// Apply modifications to the request beforehand
	preProcess(req)

	// Work around Go aggressively unescaping the URL path
	// e.g. a%2Fb and a/b would previously be indistinguishable,
	// which can cause issues.
	//
	// Consider lyrics.com/artists/AC%2FDC/
	//
	req.URL.Opaque = extractPath(req.RequestURI)

	res, err := http.DefaultTransport.RoundTrip(req)
	if err != nil {
		fmt.Printf("[%d] %s\n", http.StatusBadGateway, req.RequestURI)
		w.WriteHeader(http.StatusBadGateway)
		return
	}
	copyHeader(w.Header(), res.Header)
	w.WriteHeader(res.StatusCode)
	if res.Body != nil {
		io.Copy(w, res.Body)
	}
	fmt.Printf("[%d] %s\n", res.StatusCode, req.RequestURI)
}

func init() {
	flag.StringVar(&prefix, "prefix", "", "URL prefix to match")
	flag.StringVar(&replacement, "replacement", "", "URL prefix to replace with")
}

func main() {
	flag.Parse()
	http.ListenAndServe(":8080", new(Proxy))
}